Wall-flow filters for removing particular matter from a gaseous stream, also called filter traps, are known in the art and the general class of wall-flow filters is perhaps best represented by diesel engine filter traps. Diesel filter traps are well known in the art and some have proven to be extremely efficient at removing carbon particles (“soot”) from the exhaust of diesel engines. The most widely used type of diesel trap is the wall-flow filter which is designed to capture the soot on the wall of the filter as the exhaust passed through the porous walls of the filter. The wall-flow filter has been designed to provide for nearly complete filtration of soot without significantly hindering the exhaust flow. The material presently used to filter particulates from a diesel exhaust stream is high porosity material with mean pore sizes above 10 microns in size. These large pores effectively trap the bulk of diesel particulate emissions, including virtually all large carbonaceous particles, and they create only a modest pressure drop, not seriously affecting engine efficiency.
Unfortunately, a small percentage of particulate matter, very fine carbonaceous material, is not easily trapped by materials of relatively coarse porosity (>5 um pore size), and can escape through the filter. Although this material is a small part by weight of the total emissions, it may be significant in terms of a different measure which is the number of particles per unit volume of exhaust.
While proposals have been made regarding how to eliminate the very fine particles (for example, use of an additional very-small pore filter) such proposals have not proven practical for various reasons such as cost or the backpressure associated with very small particle filter traps. The present invention offers a reasonable solution to the problem of removing fine particles from any gaseous stream; for example, an engine exhaust gas stream.